Add xarray as requirement to build docs.
Add xarray as requirement to build docs.

csherwood-usgs/landlab,cmshobe/landlab,Carralex/landlab,landlab/landlab,RondaStrauch/landlab,RondaStrauch/landlab,Carralex/landlab,csherwood-usgs/landlab,cmshobe/landlab,amandersillinois/landlab,RondaStrauch/landlab,cmshobe/landlab,landlab/landlab,landlab/landlab,Carralex/landlab,amandersillinois/landlab